The Easy Way

Religion is a very private thing in the Nordic countries and not something you're supposed to tell others about, both because most people aren't religious so it would be considered rude, and also because they're such personal feelings for religious people.

So a sure way to ruin the mood is to wear a T-shirt with a religious message, or any other object that somehow show what religion you belong to, most of all because Nordics will probably be too polite to talk about it (like I said, you're not supposed to talk about religion), so it will be a huge elephant in the room and they will just sit and stare awkwardly at you, afraid they might say something that'll hurt your religious feelings.

Just having a modest little cross around your neck or a star of David or whatever is fine, but a T-shirt is just so in-your-face and obvious that it's sure to make people uncomfortable, 'cause let's be honest, you can wear a cross around your neck without being ultra religious, but you don't typically wear a T-shirt with a religious message unless you're VERY religious.